There are many legal results from the "HELO" command, from the RFC
HELO
S: 250
E: 500, 501, 504, 421
250 Okay (and various others)
500 Syntax error, command unrecognized
501 Syntax error in parameters or arguments
504 Command parameter not implemented
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]